Effect of relative humidity on the current-carrying tribological properties of Cu–C sliding contact pairs

Cu–C pairs are the most widely used sliding current-carrying frictional pairs owing to their good conductivity and self-lubrication ability. This study investigated the variation in the current-carrying tribological properties of Cu–C pairs and the related wear mechanism at different relative humidities. The results can provide support for carbon brush design and safe service in humid air. In this study, the current-carrying coefficient of friction and contact resistance initially increased and then decreased, reaching maximum values at a relative humidity of 30%. With an increase in the humidity from 0% to 80%, formation of a tribofilm and reduced abrasive wear were observed by scanning electron microscopy and atomic force microscopy. X-ray photoelectron spectroscopy results showed that the tribofilm on the Cu surface was a mixture of Cu, CuO, and C, and the humidity-assisted carbon transfer increased the atomic percentage of C from 60.96% to 81.13%. It was found that even for an existing tribofilm, the coefficient of friction increased immediately once the humidity support was lost. The results of this study showed that the mechanism for humidity-induced carbon lubrication was not oxidation. The lower humidity limit that could induce lubrication of the Cu–C pairs was approximately 60%. • The evolution of carbon transfer from abrasive wear to interlayer slip was observed with the increase of RH. • The results showed that the mechanism for humidity-induced carbon lubrication was not oxidation. • The lower humidity limit that could induce lubrication of the Cu–C pairs was approximately 60%.